1. Technical Field
The present invention relates generally to a display device and a method of driving the same.
2. Description of the Related Art
Organic light emitting devices (OLEDs) utilize fluorescent organic materials that emit light when excited by an electric current. These devices are an increasingly popular form of flat panel display technology due to their self-emitting properties, low power requirements, wide viewing angle, excellent responsiveness, and compatibility with full-motion video. An active matrix organic light-emitting device having a plurality of pixels arranged in matrix form realizes image display by controlling the luminance of each pixel.
Generally, an OLED has as many organic light emitting elements as the number of pixels displayed, and includes thin film transistors (TFTs) for driving the organic light emitting elements. The silicon semiconductor of a TFT is classified into two types: an amorphous silicon semiconductor (a-Si) type and a polycrystalline silicon (poly-Si) semiconductor type.
The a-Si TFTs are commonly used in the display devices utilizing glass substrates with relatively low melting points. This is because the a-Si semiconductor can be produced at a low film-forming temperature, for example, by a vapor-phase deposition method. However, the a-Si TFTs may be incompatible with the large display devices because of the relatively low field effect mobility of the a-Si TFTs. In addition, when the a-Si TFTs supply current to the organic light-emitting elements continuously, a transition of threshold voltages is generated which deteriorates the TFTs. As a result, the lifetime of the OLEDs with a-SI TFTs is reduced.
To achieve an increase in mobility, a poly-Si film can be used as the semiconductor layer of the TFTs, instead of a-Si. The poly-Si is a promising material for obtaining TFTs with high field-effect mobility, good high-frequency characteristics, and low leakage current. For example, adopting a backplane of low temperature poly-Si can extend the lifetime of the light emitting element. However, damage during the crystallization process using laser annealing technology may cause deviations of the threshold voltages between driving transistors for supplying current to the organic light emitting elements, thereby lowering uniformity of image display.
The pixel circuits that have been proposed to realize the uniform image display over the entire screen by compensating for the deviation between the threshold voltages cannot satisfy the increasing demands for high-density OLEDs, since they have too many TFTs, storage capacitors, and interconnect wiring.